1. Field of the Invention
The present invention relates generally to gyroscopes for detecting rotation of coordinates and, more particularly, to an acoustic gyroscope which utilizes a Coriolis force produced when a gas vibrated by sound is subjected to a rotational movement.
2. Description of the Related Art
Of various types of gyroscopes (which will be sometimes referred to merely as the gyros, hereinafter) for detecting rotation of coordinates, there have been put to practical use gyros of types which utilizes the effect of a spinning top, which utilizes a Coriolis force produced when a vibrating tuning fork is subjected to a rotational movement, and a laser gyro which utilizes a phase variation in light caused by the rotational movement of coordinates.
Meantime, Granqvist has disclosed in U.S. Pat. No. 2,999,389 (1961) an acoustic gyro which utilizes a Coriolis force produced when a gas vibrated by sound is subjected to a rotational movement, as shown in FIG. 12. In the drawing, the acoustic gyro includes an elongated casing 1 having a width d and a loudspeaker 2 which is provided at one end of the casing 1 to be driven by a sinusoidal oscillator 3 so that a standing wave of such a pressure distribution as shown by dotted lines is generated inside the casing 1. Connected to both side walls of the casing 1 at both sides of a node M of the standing wave are conduits 8 and 8' which guide sound pressures at their conduit inlets into a differential microphone 4. Assume now that gas particles located at the node M are sinusoidally vibrated at a velocity u(t) (expressed by the following equation (1)) along the longitudinal direction of the casing 1 by means of a sound emitted from the loudspeaker 2, that the casing 1 is rotated at an angular rate .OMEGA. in a direction shown by an arrow, and that the gas in the casing 1 has a gas density .rho.. EQU u(t)=Ucos.omega.t (1)
where t denotes time, U denotes velocity amplitude and .omega. denotes angular frequency.
Then, the Coriolis force based on the rotation of the gas causes development of a sinusoidal differential pressure (referred to as the Coriolis pressure, hereinafter) .DELTA.p(t) (expressed by the following equation (2)) which corresponds to a difference between sound pressures at the inlets of the conduits 8 and 8' and which has the same frequency as the velocity u(t) and an amplitude proportional to the quantity .OMEGA.. ##EQU1##
The Coriolis pressure .DELTA.p(t) is detected at the differential microphone 4 and sent to an amplifier 5 and then to a rectifier 6. An output of the rectifier 6, which corresponds to the magnitude of the angular rate, is indicated at an indicator 7. In the illustrated example, the conduits 8 and 8' are connected to the casing 1 at the both sides of the node M in the standing wave. This is for the purpose of detecting the Coriolis pressure .DELTA.p(t) at a position where the vibration velocity of the gas becomes maximum and the sound pressure of the standing wave for driving the gas becomes zero.
The prior art acoustic gyro explained above is featured by detecting the Coriolis pressure at the node of the standing wave. In the prior art, however, since the loudspeaker 2 as a sound source is disposed at one side of the casing 1, the mechanical characteristics of the loudspeaker 2 directly affect the acoustic characteristics of the casing 1. For example, when the stiffness of a cone of the loudspeaker 2 varies with temperatures or the like, this causes the relation between the driving sound pressure and gas velocity inside the casing 1 to be changed so that the position of the node M is shifted and thus an error occurs in the gyro output. To avoid this, in actual applications, the prior art gyro requires such control means as other control microphones to be connected to the casing 1 to maintain the amplitude U of the gas vibration velocity, apart from the microphone 4 for detection of the Coriolis pressure. However, the addition of such control means involves the complication of the gyro, thus reducing the accuracy of the gyro. In addition, even with the addition of such control means, the influence of long term drift in the sensitivity of the microphones to the output of the gyro is unavoidable. Because of such disadvantages, the acoustic gyro has not been put in practical use yet.
An acoustic gyro of a type different from the aforementioned gyro has been disclosed by Bruneau, Garing and Leblond in the J. Acoust. Soc. Am., Vol. 80, pp. 672-680, 1986, which gyro utilizes a two dimensional standing wave in a box. This gyro, however, has had the same defects as in the aforementioned prior art gyro, since the gyro also has such an arrangement that a Coriolis pressure is detected at a node of the standing wave and a loudspeaker as a sound source is disposed at one end of the box, thus disabling the realization of its practical use.